* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
#app {
  width: 300px;
  height: 300px;
  border: 1px solid black;
  margin: 50px;
}
#app header {
  display: flex;
  height: 40px;
  line-height: 40px;
  width: 100%;
}
#app header section {
  flex: 1;
  font-weight: bold;
  text-align: center;
}
#app h2 {
  text-align: center;
}
